A wireless LAN must provide a fast and reliable radio link between a base station and its terminals. A terminal in a wireless LAN is usually a portable device and therefore it is desirable to reduce the hardware requirements of the terminal as much as possible to make it inexpensive, small, and power efficient. The base station is part of the network infrastructure and usually has access to a reliable power supply so the hardware constraints on the base station are not as rigorous. Previous work (Asymmetric Equalization System for Data Transmission, Canadian patent application no. 2,234,777 published Oct. 15, 1999) has shown that radio equalization hardware can be implemented entirely in the base station, eliminating the need to put this complex and expensive hardware in every terminal. This is called an asymmetric design, because as much of the hardware as possible is moved to one side of the communication link. An asymmetric synchronization method is disclosed in this patent, which eliminates the need to put a complex synchronization circuit in every terminal of the network.
Conventional synchronization schemes will not work in an asymmetric network such as this because they always synchronize to the received signal. Suppose that a terminal has no synchronization circuit, just a free-running oscillator to control when it sends and receives information. This terminal sends information to a base station, which then recovers the timing of the signal by one of a number of conventional synchronization techniques. To send information back to the terminal, the base station must time its transmission so that the information passes through the radio channel and arrives at the terminal at the point in time when the free-running oscillator samples. The base station cannot do this because it is missing a key piece of information: the time delay between the terminal and the base station. Because it has synchronized to the received signal, it has no information about the time that the signal was transmitted and therefore cannot formulate an accurate estimate of the delay.
In order to make a synchronization system work asymmetrically, the base station must synchronize to the free-running oscillator of the terminal and not to the received signal. Once synchronized to the terminal's oscillator the base station knows the exact point in time when the terminal's transmission begins. It can then analyze the received signal to determine the absolute delay through the radio channel. Now that the base station knows the time when the terminal's free-running oscillator will sample the signal and the delay in the radio channel, it can time its transmission so that the signal arrives at the terminal already synchronized.
Knowledge of the absolute delay to the terminals may be useful to other components of the network as well since it can be directly translated into a distance measurement. A distance measurement might trigger a handoff algorithm to another network. Several distance measurements from different base stations could be used to triangulate the terminal's location.